Binding constants of various magnesium/orthophosphate complexes were experimentally determined. Precision and accuracy of intracellular pH measurements based on orthophosphate 31P NMR chemical shift were throughly analyzed in terms of intracellular magnesium ions availability and limited instrumental S/N ratio. It was demonstrated that the presence of free magnesium ions has only a marginal effect on pH-dependent intracellular orthophosphate chemical shift and no effect on derived intracellular pH estimates.